This invention relates to an improved pressure-sensitive adhesive transfer tape for use with polyethylene foam-backed absorbent products for attachment to cotton, or like, undergarments. The polyethylene foam backing is a flexible liquid-impermeable shell partially surrounding an absorbent core material.
Sanitary absorbent articles, such as sanitary napkins, conventionally comprise a liquid-impermeable backing covering at least one side of an absorbent core with a liquid-permeable facing on the side facing the fluid discharge source.
Sanitary absorbent articles such as urinary pads have been proposed in EP139,484 which use backings, flexible shells, of thermoformed polyethylene-containing polymer foam. This foam backing yields a soft, conformable article designed to withstand ordinary stress and strains and maintain a relatively large liquid storage capacity. Absorbent articles of this type are adhered to the clothing of the user conventionally with adhesive patches. These patches are placed on the liquid-impermeable backings of the absorbent articles. With absorbent articles using polyethylene-containing foam backings or shells the adhesive patches would be located on the outer face of the foam.
Placing adhesive on polyethylene-containing foam presents significant problems. Such foam has low bonding abilities to conventional adhesives. Further polyethylene containing foams are heat sensitive making hot melt coating of adhesives onto the foam an impractical option. These problems are addressed in U.S. Pat No. 4,554,191, which proposes using a double faced tape on a backing having a non-recoverable extensibility of less than 100%. The two faces of the tape are designed to have differential adhesive levels (preferably at least 1.4 to 1). The side provided with the stronger adhesive level is attached to the polyethylene-containing foam. The side with the lower level of adhesion is used to attach the absorbent article to the user's undergarment. This differential adhesion level is asserted to prevent transfer of adhesive to the undergarment or delamination of the foam. Differential levels of adhesion is allegedly accomplished using different adhesive formulations on either side of the tape backing or by masking or likewise lowering the exposed adhesive surface area on the lower adhesive level face of the tape backing. The use of the specified non-extensible backing is also stressed as critical to get acceptable performance (e.g., lack of adhesive transfer). The adhesive taught and exemplified is a A-B block copolymer elastomer tackified with solid tackifying resins. More resin is stated as needed to get the higher adhesion level formulation. The A-B block copolymer specifically described is an admixture of A-B diblock and A-B-A triblock (polystyrene-polyisoprene-polystyrene).
The problem with the U.S. Pat No. 4,554,191 approach is that the tape and its application is extremely complicated and the tape is difficult to manufacture. Conventional transfer coating tapes (a single unsupported layer of adhesive on a release surface) would be preferable in terms of simplicity. However, transfer tape methods for applying an adhesive patch are characterized as unacceptable by U.S. Pat. No. 4,554,191 for polyethylene-containing foam substrates. Transfer-applied adhesive has no backing and no inherent differential adhesion levels.